diff options
Diffstat (limited to 'third_party/rust/proc-macro-error/src/dummy.rs')
| -rw-r--r-- | third_party/rust/proc-macro-error/src/dummy.rs | 150 |
1 files changed, 150 insertions, 0 deletions
diff --git a/third_party/rust/proc-macro-error/src/dummy.rs b/third_party/rust/proc-macro-error/src/dummy.rs new file mode 100644 index 0000000000..571a595aa9 --- /dev/null +++ b/third_party/rust/proc-macro-error/src/dummy.rs @@ -0,0 +1,150 @@ +//! Facility to emit dummy implementations (or whatever) in case +//! an error happen. +//! +//! `compile_error!` does not abort a compilation right away. This means +//! `rustc` doesn't just show you the error and abort, it carries on the +//! compilation process looking for other errors to report. +//! +//! Let's consider an example: +//! +//! ```rust,ignore +//! use proc_macro::TokenStream; +//! use proc_macro_error::*; +//! +//! trait MyTrait { +//! fn do_thing(); +//! } +//! +//! // this proc macro is supposed to generate MyTrait impl +//! #[proc_macro_derive(MyTrait)] +//! #[proc_macro_error] +//! fn example(input: TokenStream) -> TokenStream { +//! // somewhere deep inside +//! abort!(span, "something's wrong"); +//! +//! // this implementation will be generated if no error happened +//! quote! { +//! impl MyTrait for #name { +//! fn do_thing() {/* whatever */} +//! } +//! } +//! } +//! +//! // ================ +//! // in main.rs +//! +//! // this derive triggers an error +//! #[derive(MyTrait)] // first BOOM! +//! struct Foo; +//! +//! fn main() { +//! Foo::do_thing(); // second BOOM! +//! } +//! ``` +//! +//! The problem is: the generated token stream contains only `compile_error!` +//! invocation, the impl was not generated. That means user will see two compilation +//! errors: +//! +//! ```text +//! error: something's wrong +//! --> $DIR/probe.rs:9:10 +//! | +//! 9 |#[proc_macro_derive(MyTrait)] +//! | ^^^^^^^ +//! +//! error[E0599]: no function or associated item named `do_thing` found for type `Foo` in the current scope +//! --> src\main.rs:3:10 +//! | +//! 1 | struct Foo; +//! | ----------- function or associated item `do_thing` not found for this +//! 2 | fn main() { +//! 3 | Foo::do_thing(); // second BOOM! +//! | ^^^^^^^^ function or associated item not found in `Foo` +//! ``` +//! +//! But the second error is meaningless! We definitely need to fix this. +//! +//! Most used approach in cases like this is "dummy implementation" - +//! omit `impl MyTrait for #name` and fill functions bodies with `unimplemented!()`. +//! +//! This is how you do it: +//! +//! ```rust,ignore +//! use proc_macro::TokenStream; +//! use proc_macro_error::*; +//! +//! trait MyTrait { +//! fn do_thing(); +//! } +//! +//! // this proc macro is supposed to generate MyTrait impl +//! #[proc_macro_derive(MyTrait)] +//! #[proc_macro_error] +//! fn example(input: TokenStream) -> TokenStream { +//! // first of all - we set a dummy impl which will be appended to +//! // `compile_error!` invocations in case a trigger does happen +//! set_dummy(quote! { +//! impl MyTrait for #name { +//! fn do_thing() { unimplemented!() } +//! } +//! }); +//! +//! // somewhere deep inside +//! abort!(span, "something's wrong"); +//! +//! // this implementation will be generated if no error happened +//! quote! { +//! impl MyTrait for #name { +//! fn do_thing() {/* whatever */} +//! } +//! } +//! } +//! +//! // ================ +//! // in main.rs +//! +//! // this derive triggers an error +//! #[derive(MyTrait)] // first BOOM! +//! struct Foo; +//! +//! fn main() { +//! Foo::do_thing(); // no more errors! +//! } +//! ``` + +use proc_macro2::TokenStream; +use std::cell::RefCell; + +use crate::check_correctness; + +thread_local! { + static DUMMY_IMPL: RefCell<Option<TokenStream>> = RefCell::new(None); +} + +/// Sets dummy token stream which will be appended to `compile_error!(msg);...` +/// invocations in case you'll emit any errors. +/// +/// See [guide](../index.html#guide). +pub fn set_dummy(dummy: TokenStream) -> Option<TokenStream> { + check_correctness(); + DUMMY_IMPL.with(|old_dummy| old_dummy.replace(Some(dummy))) +} + +/// Same as [`set_dummy`] but, instead of resetting, appends tokens to the +/// existing dummy (if any). Behaves as `set_dummy` if no dummy is present. +pub fn append_dummy(dummy: TokenStream) { + check_correctness(); + DUMMY_IMPL.with(|old_dummy| { + let mut cell = old_dummy.borrow_mut(); + if let Some(ts) = cell.as_mut() { + ts.extend(dummy); + } else { + *cell = Some(dummy); + } + }); +} + +pub(crate) fn cleanup() -> Option<TokenStream> { + DUMMY_IMPL.with(|old_dummy| old_dummy.replace(None)) +} |